The generation and use of molecular hydrogen (H2) are important processes to fundamental chemical transformations and biological functions. The overwhelming majority of systems known to either liberate or react, with H2 involve reaction at a transition metal center. Hydrogenase enzymes, as well as a plethora of synthetic stoichiometric and catalytic reagents for hydrogenation reactions, are based on the processes of oxidative addition and reductive elimination of H2 at metal center. Metal-free systems that either react with or liberate H2 are rare. A unique metal-free hydrogenase from methanogenic archaeas has been shown to catalyze reactions with H2, and theoretical studies suggest the role of a folate-like cofactor in the reversible activation/liberation of H2. Several metal-free systems have been shown to activate H2. For example, main group element-H2 reactions in low temperature matrices are also known.
Hydrogenation is the addition of hydrogen to unsaturated organic compounds. Such reactions are used for the production of a myriad of chemical products worldwide, from large-scale operations including the upgrading of crude oil and the production of bulk commodity materials to the synthesis of a variety of fine chemicals used in the food, agricultural and pharmaceutical industries. The process of hydrogen addition to unsaturated precursors is mediated by either homogeneous or heterogeneous transition metal based catalysts. In the 1960s, the advent of organometallic chemistry gave rise to homogeneous transition metal based hydrogenation catalysts for a variety of substrates. The operation of these catalysts hinges on the key step of oxidative addition of hydrogen. More recently, transition metal systems that effect heterolytic cleavage of hydrogen at a metal center have been uncovered. In these cases, a metal hydride is formed with concurrent protonation of an amido ligand.
Non-transition metal catalysts for hydrogenation reactions are all but unknown. KOtBu has been shown to act as a catalyst effecting the addition of H2 to benzophenone under forcing conditions of 200° C. and greater than 100 bar H2. Organocatalysts have been developed for hydrogenations of enones and imines; however, such systems do not employ H2 directly but rather a surrogate such as a Hantzsch ester as the stoichiometric source of hydrogen. The development of nonmetal hydrogenation catalysts hinges on the discovery of systems that react cleanly with H2, but few are known. Power and coworkers reported the hydrogenation of Ge2-alkyne analogues to give a mixture of Ge2 and primary germane products. J. W. Yang. M. T. Hechavarria Fonseca, B. List, Angew. Chem. 2004, 116, 6829; Angew. Chem. Int. Ed. 2004, 43, 6660. G. H. Spikes, J. C. Fettinger, P. P. Power, J. Am. Chem. Soc. 2005, 127, 12 232. It should be noted that non-transition metal systems have been shown to effect hydrogenation under more forcing conditions. For example, DeWitt, Ramp and Trapasso demonstrated hydrogenation with iPr3B under 67 atm 1000 psi) H2 at 220° C. E. J. DeWitt, F. L. Ramp, L. E. Trapasso, J. Am. Chem. Soc. 1961, 83, 4672-4672; F. L. Ramp, E. J. DeWitt, L. E. Trapasso, Org. Chem., 1962, 27, 4368-4372). Similarly, Haenel and coworkers (E. Osthaus, M. W. Haenel in Coal Science and Technology, Vol. 11 Elsevier, Amsterdam, 1987, pp. 765-768 (Proc. 1987 Intern. Conf. Coal Sci., Eds.: J. A. Moulijn, K. A. Nater, H. A. G. Chermin); M. Yalpani, R. Köster, M. W. Haenel, Erdoel Kohle, Erdgas, Petrochem. 1990, 43, 344-347; M. W. Haenel, J. Narangerel, U.-B. Richter, A. Rufinska, Angew. Chem. 2006, 118, 1077-1082; Angew. Chem. Int. Ed. 2006, 45, 1061-1066; M. W. Haenel, J. Narangerel, U.-B. Richter, A. Rufinska, Prep. Pap. Am. Chem. Soc., Div. Fuel Chem. 2006, 51(2), 741-742) among others showed hydrogenation of coal under almost 15 MPa and 280-350° C. using Bl3 or alkyl boranes. M. Yalpani, T. Lunow, R. Köster, Chem. Ber. 1989, 122, 687-693; (b) M. Yalpani, R. Köster. Chem. Ber. 1990, 123, 719-724. As well, superacid systems have also been shown to effect hydrogenation of alkenes using H2 pressures of at least 35 atm. M. Siskin, J. Am. Chem. Soc. 1974, 96, 3641; (b) J. Wristers. J. Am. Chem. Soc. 1975, 97, 4312.
The ability to dissociate dihydrogen represents a reaction of considerable importance in fields including hydrogenation of ethenically unsaturated feed stocks, chemical fuel storage, hydrogen purification, and hydrogen getters that prevent hydrogen levels from building beyond a preselected threshold. Traditionally, dihydrogen dissociation has involved the use of metal catalysts and in particular palladium. Conventional catalysts inclusive of metal have a number of limitations that include high material cost, high density, the heterogeneous nature of such catalysts relative to liquid phase reactants, and contamination of resultant products with metal catalysts.
Thus, there exists a need for a hydrogen dissociation catalyst that is independent of metal. Additionally, a catalyst capable of operating as a homogeneous catalyst would afford considerable operational advantages. Further, these hydrogen catalysts operate efficiently at lower or comparable temperatures to those used for existing metal based hydrogenation catalysts.